The present invention relates to a method for the preparation of a quick-curing laminate and a laminate obtainable thereby.
Plastic film is a frequently used material for the packaging of numerous articles and products. Within many fields of application, the requirements for such plastic films are of a character that cannot be met by one film alone. Consequently, various laminates of films have been developed with properties that in many respects meet the requirements better.
Usually, adhesives are used to attach separate films to each other, thus generating a laminate. Various requirements are made for such adhesives, e.g. adherence, heat resistance, transparency, and it has been proved that, inter alia, polyurethane-containing adhesives may be used for the adhesion of the separate films of a laminate, thus obtaining the requested properties of the laminate.
As far as polyurethane adhesives are concerned, it is problematic, however, that the curing time is long and that a laminate has to age, possibly at a comparatively high temperature and in a properly ventilated room, to ensure the adhesive curing.
In order to reduce the curing time of the adhesives employed, various methods have been used, including the addition of a catalyst for the curing reaction.
When using a catalyst, the simplest way to accelerate the curing of a polyurethane adhesive is to add the catalyst directly into the polyurethane adhesive, immediately before the adhesive is applied to a film that is to be attached to another film. On an industrial scale, this is not efficient, however, since to some extent the adhesive composition will have cured before use. The curing will cause an increased viscosity and a subsequently more difficult application to the film, whereby the application quantity cannot be controlled. A limited space of time is available for the administration so the adhesive composition, the so-called pot-life, and, in consequence, production disturbances may cause glue vessels and application devices having to be cleared of cured adhesives.
To overcome said problem of the low pot-life, in EP-A1-0152102 it is suggested to use a urethane adhesive containing micro capsules with a catalyst for the curing. Such micro capsules will be destroyed when the laminate film is attached by means of pressure rollers, whereby the encapsulated catalyst will be released.
The disadvantage of this method is that a homogenous distribution of the catalyst in the glue layer cannot be obtained, since the catalyst concentration will peak in the immediate vicinity of the destroyed capsule. Furthermore, such capsules will remain in the adhesive as a suspension and therefore precipitate when stored.
Another suggestion to bring a polyurethane adhesive into contact with the catalyst is disclosed in EP-A1-0586843. In this, a method for the preparation of a laminate film is disclosed, wherein at least two films are attached to each other by means of a polyurethane adhesive, the adhesive layer or the adhesive-free film being humidified with an aqueous solution bearing at least one catalyst, which accelerates the curing reaction of polyurethane adhesives. The catalyst solution humidification is problematic, however, since it will be difficult to obtain a homogenous distribution of the solution on a film, e.g. a polyethylene or polypropylene film, even when using a surfactant in the solution, inter alia, because suitable equipment for a homogenous application of the catalyst solution all over the film is not available, so that in certain sections of the prepared laminate water will accumulate, which leads to an isocyanate conversion to the corresponding amine compound, thus involving the risk of subsequent amine compound migration. Therefore, whenever preparing a laminate according to the method disclosed in EP-A1-0586843, it is necessary to age the said laminate before use, in particular when foreseen for the packaging of food.
In international patent application No. PCT/DK 96/00319, publication No. WO-A-97/03821 (which was not available to the public on the priority date of the present application), the present applicant suggested to bring the catalyst into contact with the adhesive by incorporating the catalyst in one of the films to be attached to each other by means of the adhesive. Although this reduces the curing time considerably and results in a low migration of amine compounds so far as laminates with no ink between the films are concerned, the suggestion is not recommendable when the catalyst-containing film has to be printed with ink, because the layer of ink will prevent a diffusion of catalyst from the film to the adhesive.
The object of the present invention is to provide a method for the preparation of a laminate having ink on one or both films to be attached to each other, whereby the laminate is ready for further treatment comparatively soon after its preparation, without any long-lasting ageing period.
According to the invention, this is achieved by
a) providing an ink comprising a catalyst suitable for accelerating the curing of an adhesive for films,
b) applying to a first film one or more completely or partially covering layers of the said ink,
c) laminating a second film to the first film inked surface, using an adhesive, the curing of which is accelerated by the catalyst.
The present invention relates to the laminate obtainable by this method as well. Furthermore, the invention relates to the film being coated with one or more entirely or partially covering layers of ink obtainable after step b) of the method.
Soon after the preparation of the laminate according to he invention, the measurable amine migration is comparatively low, and, in addition, a fairly satisfactory mutual adherence of the films is obtained shortly after the preparation of the laminate. Due to the quick-curing of the adhesive used as well as the low amine migration, the laminate prepared needs no or only moderate ageing before further treatment. Especially within the food industry a low amine migration is in great demand, and, when in use, the laminate should show no amine migration at all. The fact that the laminate prepared may be exposed to further treatment shortly after its preparation is advantageous from a producer""s point of view, inter alia, because this will reduce the need for storage room, and, sales of the laminate prepared can start sooner after production.
Moreover, a laminate according to the invention has a reduced tendency to telescoping, i.e. shear of the films relative to each other.
The term xe2x80x9cinkxe2x80x9d used in the present description and claims refers to both pigmented and un-pigmented inks and varnishes as well as clear and coloured varnishes. For the application of the inks methods known from the prior art, e.g. gravure, flexography or offset printing, may be used.
The ink employed may be of any suitable type, e.g. nitrocellulose, PVB, acrylic, acrylate or combinations thereof. In addition, the inks may be offset inks, e.g. oil-based or water-based. The ink may have any colour or be uncoloured.
The inks may be based on any solvent, in particular, they may be dilutable in ethanol, isopropanol, ethylacetate, methylethylketone, acetone, water or combinations thereof. Furthermore, solvent-free inks may be used.
The inks used according to the invention may be single-component, dual-component and/or even three-component inks. Single-component inks are immediately employable for printing, possibly after having been diluted by a suitable solvent. Dual-component inks are foreseen to be mixed immediately before use, whereby the first component comprises a standard ink and the second component the catalyst suitable for accelerating the curing of an adhesive for films.
Any adhesive curing in the presence of a catalyst may be used. Preferred adhesives are of the polyurethane type and alcohol-based or water-based imine-epoxy systems. Polyurethane adhesives may be single-component systems or dual-component systems. A polyurethane adhesive comprises at least one organic polyisocyanate and at least one polyol, the polyisocyanate comprising two or more isocyanate groups, and the polyol two or more hydroxy groups. The polyol may be a polyester polyol, a polyether polyol or a polyetheresterpolyol. The polyisocyanate may be a pre-polymer.
Typical examples of diisocyanates for polyurethane adhesives are 1,6-hexanediisocyanate, 1,10-decanediisocyanate, 1,3-cylopentanediisocyanate, 1,4-cyclohexanediisocyanate, 1-isocyanat-3,3,5-trimethyl-3 or 5-isocyanatomethancyclohexane, 4,4xe2x80x2-, 2,4xe2x80x2- and 2,2xe2x80x2-di-isocyanate diphenylmethane, 1,5-nanhtalenediisocyanate, 4,4xe2x80x2-diisocyanate-dicyclohexylmethane, 1,4-di-isocyanate-benzene and/or 2,4- or 2,6-diisocyanate-tolu-ene. Pre-polymers of polyisocyanates may be achieved by reacting the above diisocyanates with a polyvalent alcohol.
Typical examples of polyols for polyurethane adhesives are alkanediols with linear or branched carbon chains, e.g. ethylene glycol, 1,2-dihydroxypropane, 1,3-dihydroxypropane, 1,4-dihydroxybutane, 1,6-dihydroxyhexane and neopentylglycol. In case of the polyol being a polyetherpolyol, this is usually obtained by adding ethylene oxid to a compound comprising two or more active hydrogen atoms in a molar ratio of 30 to 90%. Suitable examples of compounds comprising two or more active hydrogen atoms are ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, glycerin, trimethylol-propane, pentaerythritol, sorbitol, saccharose or the like. Instead of ethylenoxid e.g. propylenoxid and butylenoxid may be used. In general, polyetherpolyol has a molecular mass in the range from 1,000 to 20,000, especially between 1,000 and 5,000.
Polyurethane adhesives used for single-component systems comprise e.g. pre-polymers of polyisocyanates and polyols, the said adhesive curing when reacted with water. Examples of this may be found in German patent No. 2549227 and European patent application No. 0586843 A1.
Dual-component systems either consist of pre-polymers with free isocyanate groups curing by means of a hydroxy group bearing cross-linking agent, or of pre-polymers with free hydroxy groups, curing by means of isocyanate group bearing cross-linking agents. Examples of the first type of dual-component systems are disclosed in German laid-open publication No. 4021113 and in European patent No. 150444. Examples of the latter type of dual-component systems are listed in European patent No. 176726. No matter the type of polyurethane adhesive, these may comprise a solvent, be free of solvents or water-based. For the method according to the present invention, it is preferred to use a dual-component system, in which the various components are mixed immediately before their application to the first film.
The catalysts accelerating the curing of the polyurethane adhesives are known from the prior art. Various substance groups are proved active catalysts, inter alia Lewis bases and Lewis acids. The most important Lewis bases are tertiary amines, e.g. diazabicyclooctane, triethylamine, dimethylbenzylamine, bisdimethylaminoethylether, tetramethylguanidine and bisdimethylaminomethylphenol. The most important catalytically effective Lewis acids are metallo-organic compounds, especially ten organic compounds, like tindiethylhexanoate, dibutyltindilaurate, dibutyltinbisdodecylmercaptide and lead-phenylethyldithiocarbaminate. Presumably, the metallo-organic compounds are activating the isocyanate groups thus making these electrophile, whereas the bases first and foremost increase the nucleophile properties of the OH group.
Other known catalysts are alkaline salts of organic acids and phenols, like phenol-Mannich-bases, phosphine and phospholinoxide.
Also xcex5-caprolactam has catalytic properties, and in the present invention the use of xcex5-caprolactam, which is available as a powder, flakes or in a fluid form, is preferred as a catalyst.
The catalyst itself may be an appropriate mixture of several different catalysts so as to obtain an optimal curing in relation to various types of adhesive
Usually, the catalyst content of ink is up to 20%, however, a catalyst content of approximately 5% of the solid content of the ink proved to be particularly suitable.
The catalyst, per se, may be added to the ink immediately or, as a pre-solution, optionally with use of a surfactant to obtain a homogenous distribution in the ink. The catalyst preferred, xcex5-caprolactam, is preferentially added to the ink as a pre-solution. Appropriately, the pre-solution comprises equal parts of xcex5-caprolactam and isopropanol and/or water.
The first film may partially or entirely be made of a plastic material, preferably chosen from polyesters, in particular orientated polyesters; polyolefines, e.g. polyethylenes and orientated polypropylenes or cast polypropylenes; polyamides, in particular cast polyamides or orientated polyamides; and cellophane.
The second film may be of the same type as the first film. Preferentially, however, the second film consists entirely or partially of aluminium-containing materials, metallized plastics, paper, metallized paper, or plastic materials, in particular polyethylene and polypropylene.
Both the first and the second film may incorporate a catalyst for an adhesive. Using a film with an incorporated catalyst is particularly recommendable in case of only partial application of ink to the film. In this way, a catalyzed curing also takes place outside the areas of the film covered with catalyst-containing ink.
In cases when a film without an incorporated catalyst is used along with partial application of a catalyst-containing coloured ink it is recommendable to apply a completely covering layer of clear varnish or unpigmented catalyst-containing ink to the first film. In this way, a catalyzed curing is ensured outside the areas of the film originally covered with the coloured ink as well.
Multi-colour printing is well-known in the prior art, i.e. applying several layers of different inks consecutively or on top of each other. All applied inks may contain a catalyst, so that catalyzed curing is taking place within all areas of the laminate. If at last a completely covering ink is applied, a so-called background colour, only that ink needs to contain a catalyst to ensure a catalyzed curing within all areas of the laminate.
The first and/or the second film may be a laminate, per se. In particular when using a metallo-film in the laminate, it may be advantageous to coat the metallo-film with a plastic f prior to applying the ink.
When the catalyst-containing ink has been applied to the first film this is laminated to the second film. The adhesive may either be applied to the first or the second film, prior to their being attached together or be added simultaneously. Preferably, the adhesive is applied to the first ink covered film.
The laminates prepared may be used for the wrapping and packaging of food, i.e. they get into immediate contact with food. Therefore, it must be ensured that no noxious substances are transferred to the food, or at least the extent of the transfer must be non-toxic. xcex5-caprolactam is a comparatively non-toxic compound, which is released for the market by the authorities for use in materials getting into contact with food. Since the catalyst is placed between the two films of the laminate, it will rot set into immediate contact with the food.
Without any intention of limiting the invention to any specific theoretical explanation, it is assumed, that the catalyst in the ink diffuses and penetrates the adhesive layer thus accelerating the curing of the adhesive.
In the following, the present invention will be illustrated by means of examples, however, these examples are not to be considered as a limitation of the scope of the protection.